1. Field of the Invention
This invention relates to a method and apparatus for servicing a steam generator and, more particularly, to a method and apparatus for remotely servicing the tubes and tubesheet of such a generator.
2. Prior Art
In a pressurized water nuclear powered electric generating system, the heat generated by the nuclear reaction is absorbed by a primary coolant that circulates through the reactor core and is utilized to generate steam in a steam generator. The steam generator typically is an upright cylindrical pressure vessel with hemispherical end sections. A transverse plate called a tubesheet, located at the lower end of the cylindrical section, divides the steam generator into a primary side, which is the lower hemispherical section below the tubesheet, and a secondary side above the tubesheet. A vertical wall bisects the primary side into an inlet section and an outlet section. The tubesheet is a thick carbon steel plate with an array of thousands of holes into which are inserted the ends of U-shaped tubes. One end of each U-shaped tube is inserted into a hole in the tubesheet which communicates with the inlet section of the primary side and the other end is inserted in a hole which communicates with the outlet section. The primary coolant is introduced under pressure into the inlet section of the primary side, circulates through the U-shaped tubes and exits through the outlet section. Water introduced into the secondary side of the steam generator circulates around the U-shaped tubes and is transformed into steam by heat given up by the primary coolant.
Occasionally during the operation of the steam generator, leaks develop in some of the tubes. This is undesirable because the primary coolant is radioactive and any cross-feed of reactor coolant into the secondary side of the generator contaminates the steam. It is not practical, however, to replace leaky tubing as it occurs, but instead the steam generator is taken out of service temporarily and the affected tubes are plugged at both ends. In view of the thousands of tubes in the steam generator, plugging of a few does not appreciably affect the efficiency of heat transfer.
Eventually, however, a sufficient number of tubes may be plugged to adversely affect heat transfer and generator efficiency. More often, the steam generator is shut down for scheduled retubing of the entire unit. In the retubing process, all the tube holes, including any plugged holes, are drilled out and spot-faced from the primary side and the tubes are then pulled out from the secondary side. New tubes are inserted from the secondary side with tube guides inserted in the tube ends to ease their passage through holes in transverse support plates on the secondary side and the appropriate holes in the inlet and outlet sides of the tubesheet. The tube guides are then removed from the primary side and the ends of the tubes are aligned with the spot-faced end of the hole in the tubesheet, tack rolled and then welded in place.
While space to maneuver is not a particular problem on the secondary side of the steam generator, the radius of the partitioned, hemispherical primary side is typically approximately five feet which does not provide much working room especially near the circumference of the tubesheet. In addition, the primary side is radioactive which requires worker protection and limitation of exposure time.
In an initial attempt to at least partially automate refurbishing of steam generators, a tool fixture was developed which cam locked into holes in the tubesheet to support a tool with an automatic feed. Only the drilling and spot-facing operation and the welding were performed by this unit and a worker was required to enter the confined primary side of the generator to move the fixture from one hole set to the next. In view of the thousands of holes in the typical tubesheet, this procedure was very time consuming.
Subsequently, a fixture was developed which can be "walked" from hole to hole by an operator outside the steam generator. In this machine, the operator manipulates a scale model to move the cam locks from one hole to another by reference to a television screen. While this machine speeds up the drilling/spot-facing operation and the welding and reduces worker exposure to radioactivity, it requires a skilled operator, still takes longer than desirable and does not perform all of the required operations so that a workman must still spend a considerable amount of time in the primary side of the generator. In addition, with both of these prior art fixtures the tube holes are spot-faced to a depth which is referenced to the face of the tubesheet adjacent the hole and not to a common plane.
It is a primary object of this invention to provide a method and apparatus which automates all the retubing operations to minimize downtime and worker exposure to radiation.
It is another object of this invention to provide such a method and apparatus which accommodates for the imprecise location of tube holes and provides a map of the exact locations.
It is yet another object of this invention to provide such a method and apparatus which accommodates for imperfections in the flatness of the tubesheet surface.
It is a more specific objective of the invention to provide a method and apparatus such as in the previous object for spot-facing the tube holes to a common plane despite imperfections in flatness of the tubesheet.
It is still another object of the invention to provide a method and apparatus which coordinates retubing operations on both the inlet and outlet sections of the steam generator primary side.
It is also an object of this invention to provide a method and apparatus for precise alignment of the apparatus with the tubesheet, taking into account imprecise orientation of the tubesheet in the steam generator.